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United States Patent |
5,690,803
|
Castegnier
|
November 25, 1997
|
Method of enhancing transfer of coagulated colloid onto a substrate
during electrocoagulation printing
Abstract
An improved electrocoagulation printing method comprising the steps of (a)
providing a positive electrode formed of an electrolytically inert metal
and having a continuous passivated surface moving at substantially
constant speed along a predetermined path, the passivated surface defining
a positive electrode active surface; (b) forming on the positive electrode
active surface a plurality of dots of colored, coagulated colloid
representative of a desired image, by electrocoagulation of an
electrolytically coagulable colloid present in an electrocoagulation
printing ink comprising a liquid colloidal dispersion containing the
electrolytically coagulable colloid, a dispersing medium, a soluble
electrolyte and a coloring agent; and (c) bringing a substrate into
contact with the dots of colored, coagulated colloid to cause transfer of
the colored, coagulated colloid from the positive electrode active surface
onto the substrate and thereby imprint the substrate with the image. The
improvement resides in maintaining the positive electrode active surface
and the ink at a temperature of about 35.degree. C. to about 60.degree. C.
to increase the viscosity of the coagulated colloid in step (b) so that
the dots of colored, coagulated colloid remain coherent during their
transfer in step (c), thereby enabling the colored, coagulated colloid to
be substantially completely transferred onto the substrate in step (c).
Inventors:
|
Castegnier; Adrien (Outremont, CA)
|
Assignee:
|
Elcorsy Technology Inc. (Saint-Laurent, CA)
|
Appl. No.:
|
789157 |
Filed:
|
January 27, 1997 |
Current U.S. Class: |
204/486; 101/DIG.29; 204/483; 204/508 |
Intern'l Class: |
C25D 013/04 |
Field of Search: |
204/486,483,508
101/DIG. 29
|
References Cited
U.S. Patent Documents
3010883 | Nov., 1961 | Johnson et al. | 204/18.
|
3145156 | Aug., 1964 | Oster | 204/180.
|
4661222 | Apr., 1987 | Castegnier | 204/180.
|
4895629 | Jan., 1990 | Castegnier | 204/180.
|
5449392 | Sep., 1995 | Castegnier et al. | 218/46.
|
5538601 | Jul., 1996 | Castegnier | 204/486.
|
Primary Examiner: Gorgos; Kathryn L.
Assistant Examiner: Mayekar; Kishor
Attorney, Agent or Firm: Swabey Ogilvy Renault
Claims
I claim:
1. In an electrocoagulation printing method comprising the steps of:
a) providing a positive electrode formed of an electrolytically inert metal
and having a continuous passivated surface moving at a constant speed
along a selected path, said passivated surface defining a positive
electrode active surface;
b) forming on said positive electrode active surface a plurality of dots of
colored, coagulated colloid representative of a desired image, by
electrocoagulation of an electrolytically coagulable colloid present in an
electrocoagulation printing ink comprising a liquid colloidal dispersion
containing said electrolytically coagulable colloid, a dispersing medium,
a soluble electrolyte and a coloring agent; and
c) bringing a substrate into contact with the dots of colored, coagulated
colloid to cause transfer of the dots of colored, coagulated colloid from
the positive electrode active surface onto said substrate and to imprint
said substrate with said image;
the improvement which comprises maintaining said positive electrode active
surface and said ink at a temperature of about 35.degree. C. to about
60.degree. C. to increase viscosity of the coagulated colloid in step (b)
said that the dots of colored, coagulated colloid remain coherent during
transfer in step (c) to enable the colored, coagulated colloid to be
substantially completely transferred onto said substrate in step (c).
2. A method as claimed in claim 1, wherein the temperature of said positive
electrode active surface and said ink is about 40.degree. C.
3. A method as claimed in claim 1, wherein said ink is maintained at said
temperature by heating said positive electrode active surface and applying
said ink on the heated electrode surface to cause a transfer of heat
therefrom to said ink.
4. A method as claimed in claim 1, wherein said dispersing medium is water
and said electrolyte is selected from the group consisting of alkali metal
halides and alkaline earth metal halides.
5. A method as claimed in claim 4, wherein said electrolyte is present in
said ink in an amount of about 4.5 to about 6% by weight, based on the
total weight of the ink.
6. A method as claimed in claim 5, wherein said electrolyte is potassium
chloride.
7. A method as claimed in claim 4, wherein said substrate is a water
absorbent paper.
8. A method as claimed in claim 7, wherein said water absorbent paper has a
thickness of about 60 .mu.m to about 70 .mu.m.
9. A method as claimed in claim 1, wherein steps (b) and (c) are repeated
several times to define a corresponding number of printing stages arranged
at selected locations along said path and each using a coloring agent of
different color, and to produce several differently colored images of
coagulated colloid which are transferred at respective transfer positions
onto said substrate in superimposed relation to provide a polychromic
image.
10. A method as claimed in claim 9, wherein said positive electrode is a
cylindrical electrode having a central longitudinal axis and rotating at
said constant speed about said longitudinal axis, and wherein said
printing stages are arranged around said positive cylindrical electrode.
11. A method as claimed in claim 10, wherein step (b) is carried out by:
i) providing a plurality of negative electrolytically inert electrodes
electrically insulated from one another and arranged in rectilinear
alignment to define a series of corresponding negative electrode active
surfaces disposed in a plane parallel to the longitudinal axis of said
positive electrode and spaced from the positive electrode active surface
by a constant selected gap, said negative electrodes being spaced from one
another by a distance at least equal to said electrode gap;
ii) coating the positive electrode active surface with an olefinic
substance and a metal oxide to form on said surface micro-droplets of
olefinic substance containing the metal oxide;
iii) filling said electrode gap with said electrocoagulation printing ink;
iv) electrically energizing selected ones of said negative electrodes to
cause point-by-point selective coagulation and adherence of the colloid
onto the olefin and metal oxide-coated positive electrode active surface
opposite the electrode active surfaces of said energized negative
electrodes while said positive electrode is rotating to form said dots of
colored, coagulated colloid; and
v) removing any remaining non-coagulated colloid from said positive
electrode active surface.
12. A method as claimed in claim 11, wherein step (b) (ii) is carried out
by providing a distribution roller extending parallel to said positive
electrode and having a peripheral coating comprising an oxide ceramic
material, applying said olefinic substance in the form of an oily
dispersion containing said metal oxide as dispersed phase onto the ceramic
coating to form on a surface thereof a film of said oily dispersion
uniformly covering the surface of said ceramic coating, said film of oily
dispersion breaking down into micro-droplets containing said olefinic
substance in admixture with said metal oxide and having substantially
uniform size and distribution, and transferring said micro-droplets from
said ceramic coating onto said positive electrode active surface.
13. A method as claimed in claim 12, wherein said oxide ceramic material
comprises a fused mixture of alumina and titania.
14. A method as claimed in claim 12, wherein said oily dispersion is
applied onto said ceramic coating by disposing an applicator roller
parallel to said distribution roller and in pressure contact engagement
therewith to form a first nip, and rotating said applicator roller and
said distribution roller in register while feeding said oily dispersion
into said first nip, such that said oily dispersion upon passing through
said first nip forms said film uniformly covering the surface of said
ceramic coating.
15. A method as claimed in claim 14, wherein said micro-droplets are
transferred from said distribution roller to said positive electrode by
disposing a transfer roller parallel to said distribution roller and in
contact engagement therewith to form a second nip, positioning said
transfer roller in pressure contact engagement with said positive
electrode to form a third nip, and rotating said transfer roller and said
positive electrode in register for transferring said micro-droplets from
said distribution roller to said transfer roller at said second nip and
thereafter transferring said micro-droplets from said transfer roller to
said positive electrode at said third nip.
16. A method as claimed in claim 15, wherein said applicator roller and
said transfer roller are each provided with a peripheral covering of a
resilient material which is resistant to attack by said olefinic
substance.
17. A method as claimed in claim 11, wherein step (b) (ii) is carried out
by providing first and second distribution rollers extending parallel to
said positive electrode and each having a peripheral coating comprising an
oxide ceramic material, applying said olefinic substance in the form of an
oily dispersion containing said metal oxide as dispersed phase onto the
ceramic coating of said first distribution roller to form on a surface
thereof a film of said oily dispersion uniformly covering the surface of
said ceramic coating, said film of oily dispersion at least partially
breaking down into micro-droplets containing said olefinic substance in
admixture with said metal oxide and having substantially uniform size and
distribution, transferring the at least partially broken film from said
first distribution roller to said second distribution roller to cause said
film to substantially completely break on the ceramic coating of said
second distribution roller into said micro-droplets having substantially
uniform size and distribution, and transferring said micro-droplets from
the ceramic coating of said second distribution roller onto said positive
electrode active surface.
18. A method as claimed in claim 17, wherein the ceramic coatings of said
first distribution roller and said second distribution roller comprise the
same oxide ceramic material, and wherein said oxide ceramic material
comprises a fused mixture of alumina and titania.
19. A method as claimed in claim 17, wherein said oily dispersion is
applied onto the ceramic coating of said first distribution roller by
disposing an applicator roller parallel to said first distribution roller
and in pressure contact engagement therewith to form a first nip, and
rotating said applicator roller and said first distribution roller in
register while feeding said oily dispersion into said first nip, such that
said oily dispersion upon passing through said first nip forms said film
uniformly covering the surface of said ceramic coating.
20. A method as claimed in claim 19, wherein said at least partially broken
film of oily dispersion is transferred from said first distribution roller
to said second distribution roller and said micro-droplets are transferred
from said second distribution roller to said positive electrode by
disposing a first transfer roller between said first distribution roller
and said second distribution roller in parallel relation thereto,
positioning said first transfer roller in pressure contact engagement with
said first distribution roller to form a second nip and in contact
engagement with said second distribution roller to form a third nip,
rotating said first distribution roller and said first transfer roller in
register for transferring said at least partially broken film from said
first distribution roller to said first transfer roller at said second
nip, disposing a second transfer roller parallel to said second
distribution roller and in pressure contact engagement therewith to form a
fourth nip, positioning said second transfer roller in pressure contact
engagement with said positive electrode to form a fifth nip, and rotating
said second distribution roller, said second transfer roller and said
positive electrode in register for transferring said at least partially
broken film from said first transfer roller to said second distribution
roller at said third nip, then transferring said micro-droplets from said
second distribution roller to said second transfer roller at said fourth
nip and thereafter transferring said micro-droplets from said second
transfer roller to said positive electrode at said fifth nip.
21. A method as claimed in claim 20, wherein said applicator roller, said
first transfer roller and said second transfer roller are each provided
with a peripheral covering of a resilient material which is resistant to
attack by said olefinic substance.
22. A method as claimed in claim 11, further including the step of
polishing the olefin and metal oxide-coated positive electrode active
surface to increase adherence of said micro-droplets onto said positive
electrode active surface, prior to step (b) (iii) of each printing stage.
23. A method as claimed in claim 11, wherein said olefinic substance is
selected from the group consisting of arachidonic acid, oleic acid,
linoleic acid, linolenic acid, palmitoleic acid, corn oil, linseed oil,
olive oil, peanut oil, soybean oil and sunflower oil, and wherein said
metal oxide is selected from the group consisting of aluminum oxide, ceric
oxide, chromium oxide, cupric oxide, magnesium oxide, manganese oxide,
titanium dioxide and zinc oxide.
24. A method as claimed in claim 23, wherein said metal oxide is present in
said oily dispersion in an amount of about 15 to about 40% by weight,
based on the total weight of the dispersion.
25. A method as claimed in claim 23, wherein said olefinic substance is
oleic acid or linoleic acid and said metal oxide is chromium oxide.
26. A method as claimed in claim 25, wherein said oily dispersion contains
about 75 wt. % of oleic acid or linoleic acid and about 25 wt. % of
chromium oxide.
27. A method as claimed in claim 10, wherein the temperature of said
positive electrode active surface and said ink is about 40.degree. C.
28. A method as claimed in claim 10, wherein said ink is maintained at said
temperature by heating said positive electrode active surface and applying
said ink on the heated electrode surface to cause a transfer of heat
therefrom to said ink.
29. A method as claimed in claim 10, wherein said dispersing medium is
water and said electrolyte is selected from the group consisting of alkali
metal halides and alkaline earth metal halides.
30. A method as claimed in claim 29, wherein said electrolyte is present in
said ink in an amount of about 4.5 to about 6% by weight, based on the
total weight of the ink.
31. A method as claimed in claim 30, wherein said electrolyte is potassium
chloride.
32. A method as claimed in claim 29, Wherein said substrate is a water
absorbent paper.
33. A method as claimed in claim 32, wherein said water absorbent paper has
a thickness of about 60 .mu.m to about 70 .mu.m.
34. A method as claimed in claim 10, wherein said substrate is in the form
of a continuous web which is passed through said respective transfer
positions for being imprinted with said colored images at said printing
stages.
35. A method as claimed in claim 34, wherein step (c) is carried out by
providing at each transfer position a pressure roller extending parallel
to said positive electrode and in pressure contact engagement therewith to
form a nip and permit said pressure roller to be driven by said positive
electrode upon rotation thereof, and guiding said web to pass through said
nip.
36. A method as claimed in claim 35, wherein each said pressure roller is
provided with a peripheral covering of a synthetic rubber material.
37. A method as claimed in claim 36, wherein said synthetic rubber material
comprises a polyurethane having a Shore A hardness of about 95.
38. A method as claimed in claim 10, further including the step of removing
after step (c) of each printing stage any remaining coagulated colloid
from said positive electrode active surface.
39. A method as claimed in claim 38, wherein said positive electrode is
rotatable in a selected direction and wherein any remaining coagulated
colloid is removed from said positive electrode active surface by
providing an elongated rotatable brush extending parallel to the
longitudinal axis of said positive electrode, said brush being provided
with a plurality of radially extending bristles having extremities
contacting said positive electrode active surface, rotating said brush in
a direction opposite to the direction of rotation of said positive
electrode to cause said bristles to frictionally engage said positive
electrode active surface, and directing jets of cleaning liquid under
pressure against said positive electrode active surface, from either side
of said brush.
40. A method as claimed in claim 39, wherein said positive electrode active
surface and said ink are maintained at said temperature by heating said
cleaning liquid to heat said positive electrode active surface upon
contacting same and applying said ink on the heated electrode surface to
cause a transfer of heat therefrom to said ink.
Description
BACKGROUND OF THE INVENTION
The present invention pertains to improvements in the field of
electrocoagulation printing. More particularly, the invention relates to a
method of enhancing transfer of coagulated colloid onto a substrate during
electrocoagulation printing.
In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, Applicant has described a
high-speed electrocoagulation printing method and apparatus in which use
is made of a positive electrode in the form of a revolving cylinder having
a passivated surface onto which dots of colored, coagulated colloid
representative of an image are produced. These dots of colored, coagulated
colloid are thereafter contacted with a substrate such as paper to cause
transfer of the colored, coagulated colloid onto the substrate and thereby
imprint the substrate with the image. As explained in this patent, the
positive electrode is coated with a dispersion containing an olefinic
substance and a metal oxide prior to electrical energization of the
negative electrodes in order to weaken the adherence of the dots of
coagulated colloid to the positive electrode and also to prevent an
uncontrolled corrosion of the positive electrode. In addition, gas
generated as a result of electrolysis upon energizing the negative
electrodes is consumed by reaction with the olefinic substance so that
there is no gas accumulation between the negative and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined
between the positive and negative electrodes consists essentially of a
liquid colloidal dispersion containing an electrolytically coagulable
colloid, a dispersing medium, a soluble electrolyte and a coloring agent.
Where the coloring agent used is a pigment, a dispersing agent is added
for uniformly dispersing the pigment into the ink. After coagulation of
the colloid, any remaining non-coagulated colloid is removed from the
surface of the positive electrode, for example, by scraping the surface
with a soft rubber squeegee, so as to fully uncover the colored,
coagulated colloid which is thereafter transferred onto the substrate. The
surface of the positive electrode is thereafter cleaned by means of a
plurality of rotating brushes and a cleaning liquid to remove any residual
coagulated colloid adhered to the surface of the positive electrode.
When a polychromic image is desired, the negative and positive electrodes,
the positive electrode coating device, ink injector, rubber squeegee and
positive electrode cleaning device are arranged to define a printing unit
and several printing units each using a coloring agent of different color
are disposed in tandem relation to produce several differently colored
images of coagulated colloid which are transferred at respective transfer
Stations onto the substrate in superimposed relation to provide the
desired polychromic image. Alternatively, the printing units can be
arranged around a single roller adapted to bring the substrate into
contact with the dots of colored, coagulated colloid produced by each
printing unit, and the substrate which is in the form of a continuous web
is partially wrapped around the roller and passed through the respective
transfer stations for being imprinted with the differently colored images
in superimposed relation.
The electrocoagulation printing method described in the aforementioned U.S.
Pat. No. 4,895,629 is carried out at room temperature which is generally
about 25.degree.-30.degree. C. Applicant has observed that most of the
papers used as substrates for electrocoagulation printing had to be
humidified with a mist of water in order to prevent the dots of colored,
coagulated colloid from breaking apart when being transferred from the
positive electrode active surface onto the paper. Without paper
humidification, only about 60-70% of the colored, coagulated colloid were
transferred onto dry paper, a substantial amount of the coagulated colloid
remaining on the positive electrode surface.
When using a water absorbent paper, a slight humidification of the paper
caused up to about 90% of the colored, coagulated colloid to be
transferred. However, the humidified paper suffered a reduction in
mechanical strength. Other problems encountered with paper humidification
were a reduction in the transfer of high optical densities as well as the
formation of undesirable colored background on the printed image.
Humidification of newspaper which is a thin water absorbent paper having a
thickness of about 60-70 .mu.m was also impossible since newspaper cannot
sustain humidification without tearing.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome the above
drawbacks and to provide a method of enhancing transfer of a coagulated
colloid onto a substrate during electrocoagulation printing.
In accordance with the present invention, there is provided an improved
electrocoagulation printing method comprising the steps of:
a) providing a positive electrode formed of an electrolytically inert metal
and having a continuous passivated surface moving at substantially
constant speed along a predetermined path, the passivated surface defining
a positive electrode active surface;
b) forming on the positive electrode active surface a plurality of dots of
colored, coagulated colloid representative of a desired image, by
electrocoagulation of an electrolytically coagulable colloid present in an
electrocoagulation printing ink comprising a liquid colloidal dispersion
containing the electrolytically coagulable colloid, a dispersing medium, a
soluble electrolyte and a coloring agent; and
c) bringing a substrate into contact with the dots of colored, coagulated
colloid to cause transfer of the colored, coagulated colloid from the
positive electrode active surface onto the substrate and thereby imprint
the substrate with the image;
the improvement which comprises maintaining the positive electrode active
surface and the ink at a temperature of about 35.degree. C. to about
60.degree. C. to increase the viscosity of the coagulated colloid in step
(b) so that the dots of colored, coagulated colloid remain coherent during
their transfer in step (c), thereby enabling the colored, coagulated
colloid to be substantially completely transferred onto the substrate in
step (c).
It has surprisingly been found, according to the invention, that by
increasing the temperature of the positive electrode active surface as
well as the temperature of the ink to within the range of from about
35.degree. C. to about 60.degree. C., and preferably to about 40.degree.
C., the viscosity of the coagulated colloid in step (b) increases and that
such a viscosity increase permits the dots of colored, coagulated colloid
to remain coherent during their transfer in step (c). Accordingly, no
humidification of the substrate is required. Where the ink contains water
as the medium for dispersing the colloid and the substrate used is a thin
water absorbent paper such as newspaper, about 90 to 95 wt. % of the
colored, coagulated colloid can thus be transferred onto such a substrate.
Moreover, at a temperature of 35.degree.-60.degree. C., Applicant has
observed that there is no formation of any undesirable colored background
on the printed image. If the temperature of the positive electrode active
surface and the ink is below 35.degree. C., the viscosity of the
coagulated colloid is not sufficient for the dots of colored, coagulated
colloid to remain coherent. At a temperature above 60.degree. C., problems
such as condensation of water vapor on the equipment are encountered.
DESCRIPTION OF PREFERRED EMBODIMENTS
Where a polychromic image is desired, steps (b) and (c) of the above
electrocoagulation printing method are repeated several times to define a
corresponding number of printing stages arranged at predetermined
locations along the aforesaid path and each using a coloring agent of
different color, and to thereby produce several differently colored images
of coagulated colloid which are transferred at the respective transfer
positions onto the substrate in superimposed relation to provide a
polychromic image.
The positive electrode used can be in the form of a moving endless belt as
described in Applicant's U.S. Pat. No. 4,661,222, or in the form of a
revolving cylinder as described in the aforementioned U.S. Pat. No.
4,895,629 or in Applicant's U.S. Pat. No. 5,538,601, the teachings of
which are incorporated herein by reference. In the later case, the
printing stages are arranged around the positive cylindrical electrode.
Preferably, the positive electrode active surface and the ink are
maintained at a temperature of about 35.degree.-60.degree. C. by heating
the positive electrode active surface and applying the ink on the heated
electrode surface to cause a transfer of heat therefrom to the ink.
When use is made of a positive electrode of cylindrical configuration
rotating at substantially constant speed about its central longitudinal
axis, step (b) of the above electrocoagulation printing method is carried
out by:
i) providing a plurality of negative electrolytically inert electrodes
electrically insulated from one another and arranged in rectilinear
alignment to define a series of corresponding negative electrode active
surfaces disposed in a plane parallel to the longitudinal axis of the
positive electrode and spaced from the positive electrode active surface
by a constant predetermined gap, the negative electrodes being spaced from
one another by a distance at least equal to the electrode gap;
ii) coating the positive electrode active surface with an olefinic
substance and a metal oxide to form on the surface micro-droplets of
olefinic substance containing the metal oxide;
iii) filling the electrode gap with the aforesaid electrocoagulation
printing ink;
iv) electrically energizing selected ones of the negative electrodes to
cause point-by-point selective coagulation and adherence of the colloid
onto the olefin and metal oxide-coated positive electrode active surface
opposite the electrode active surfaces of the energized negative
electrodes while the positive electrode is rotating, thereby forming the
dots of colored, coagulated colloid; and
v) removing any remaining non-coagulated colloid from the positive
electrode active surface.
As explained in U.S. Pat. No. 4,895,629, spacing of the negative electrodes
from one another by a distance which is equal to or greater than the
electrode gap prevents the negative electrodes from undergoing edge
corrosion. On the other hand, coating of the positive electrode with an
olefinic substance and a metal oxide prior to electrical energization of
the negative electrodes weakens the adherence of the dots of coagulated
colloid to the positive electrode and also prevents an uncontrolled
corrosion of the positive electrode. In addition, gas generated as a
result of electrolysis upon energizing the negative electrodes is consumed
by reaction with the olefinic substance so that there is no gas
accumulation between the negative and positive electrodes.
Examples of suitable electrolytically inert metals from which the positive
and negative electrodes can be made are stainless steel, platinum,
chromium, nickel and aluminum. The positive electrode is preferably made
of stainless steel, aluminum or tin so that upon electrical energization
of the negative electrodes, dissolution of the passive oxide film on such
an electrode generates trivalent ions which then initiate coagulation of
the colloid.
The gap which is defined between the positive and negative electrodes can
range from about 50 .mu.m to about 100 .mu.m, the smaller the electrode
gap the sharper are the dots of coagulated colloid produced. Where the
electrode gap is of the order of 50 .mu.m, the negative electrodes are
preferably spaced from one another by a distance of about 75 .mu.m.
Examples of suitable olefinic substances which may be used to coat the
surface of the positive electrode in step (b)(ii) include unsaturated
fatty acids such as arachidonic acid, linoleic acid, linolenic acid, oleic
acid and palmitoleic acid and unsaturated vegetable oils such as corn oil,
linseed oil, olive oil, peanut oil, soybean oil and sunflower oil. The
olefinic substance is advantageously applied onto the positive electrode
active surface in the form of an oily dispersion containing the metal
oxide as dispersed phase. Examples of suitable metal oxides include
aluminum oxide, ceric oxide, chromium oxide, cupric oxide, magnesium
oxide, manganese oxide, titanium dioxide and zinc oxide; chromium oxide is
the preferred metal oxide. Depending on the type of metal oxide used, the
amount of metal oxide may range from about 15 to about 40% by weight,
based on the total weight of the dispersion. A particularly preferred
dispersion contains about 75 wt. % of oleic acid or linoleic acid and
about 25 wt. % of chromium oxide. Operating at a temperature of about
35.degree.-60.degree. C. enables one to lower the concentration of metal
oxide in the oily dispersion and thus to reduce wear of the positive
electrode active surface.
The oily dispersion containing the olefinic substance and the metal oxide
is advantageously applied onto the positive electrode active surface by
providing a distribution roller extending parallel to the positive
cylindrical electrode and having a peripheral coating comprising an oxide
ceramic material, applying the oily dispersion onto the ceramic coating to
form on a surface thereof a film of the oily dispersion uniformly covering
the surface of the ceramic coating, the film of oily dispersion breaking
down into micro-droplets containing the olefinic substance in admixture
with the metal oxide and having substantially uniform size and
distribution, and transferring the micro-droplets from the ceramic coating
onto the positive electrode active surface. As explained in Applicant's
U.S. Pat. No. 5,449,392 of Sep. 12, 1995, the teaching of which is
incorporated herein by reference, the use of a distribution roller having
a ceramic coating comprising an oxide ceramic material enables one to form
on a surface of such a coating a film of the oily dispersion which
uniformly covers the surface of the ceramic coating and thereafter breaks
down into micro-droplets containing the olefinic substance in admixture
with the metal oxide and having substantially uniform size and
distribution. The micro-droplets formed on the surface of the ceramic
coating and transferred onto the positive electrode active surface
generally have a size ranging from about 1 to about 5.mu..
A particularly preferred oxide ceramic material forming the aforesaid
ceramic coating comprises a fused mixture alumina and titania. Such a
mixture may comprise about 60 to about 90 weight % of alumina and about 10
to about 40 weight % of titania.
According to a preferred embodiment of the invention, the oily dispersion
is applied onto the ceramic coating by disposing an applicator roller
parallel to the distribution roller and in pressure contact engagement
therewith to form a first nip, and rotating the applicator roller and the
distribution roller in register while feeding the oily dispersion into the
first nip, whereby the oily dispersion upon passing through the first nip
forms a film uniformly covering the surface of the ceramic coating. The
micro-droplets are advantageously transferred from the distribution roller
to the positive electrode by disposing a transfer roller parallel to the
distribution roller and in contact engagement therewith to form a second
nip, positioning the transfer roller in pressure contact engagement with
the positive electrode to form a third nip, and rotating the transfer
roller and the positive electrode in register for transferring the
micro-droplets from the distribution roller to the transfer roller at the
second nip and thereafter transferring the micro-droplets from the
transfer roller to the positive electrode at the third nip. Such an
arrangement of rollers is described in the aforementioned U.S. Pat. No.
5,449,392.
Preferably, the applicator roller and the transfer roller are each provided
with a peripheral covering of a resilient material which is resistant to
attack by the olefinic substance, such as a synthetic rubber material. For
example, use can be made of a polyurethane having a Shore A hardness of
about 50 to about 70 in the case of the applicator roller, or a Shore A
hardness of about 60 to about 80 in the case of the transfer roller.
In some instances, depending on the type of olefinic substance used,
Applicant has noted that the film of oily dispersion only partially breaks
down on the surface of the ceramic coating into the desired
micro-droplets. Thus, in order to ensure that the film of oily dispersion
substantially completely breaks on the ceramic coating into micro-droplets
of olefinic substance containing the metal oxide and having substantially
uniform size and distribution, step (b)(ii) of the electrocoagulation
printing method of the invention is preferably carried out by providing
first and second distribution rollers extending parallel to the positive
cylindrical electrode and each having a peripheral coating comprising an
oxide ceramic material, applying the oily dispersion onto the ceramic
coating of the first distribution roller to form on a surface thereof a
film of the oily dispersion uniformly covering the surface of the ceramic
coating, the film of oily dispersion at least partially breaking down into
micro-droplets containing the olefinic substance in admixture with the
metal oxide and having substantially uniform size and distribution,
transferring the at least partially broken film from the first
distribution roller to the second distribution roller so as to cause the
film to substantially completely break on the ceramic coating of the
second distribution roller into the desired micro-droplets having
substantially uniform size and distribution, and transferring the
micro-droplets from the ceramic coating of the second distribution roller
onto the positive electrode active surface. Preferably, the ceramic
coatings of the first distribution roller and the second distribution
roller comprise the same oxide ceramic material. Such an arrangement of
rollers is described in Applicant's U.S. Pat. No. 5,538,601 of Jul. 23,
1996, the teaching of which is incorporated herein by reference.
According to a preferred embodiment, the oily dispersion is applied onto
the ceramic coating of the first distribution roller by disposing an
applicator roller parallel to the first distribution roller and in
pressure contact engagement therewith to form a first nip, and rotating
the applicator roller and the first distribution roller in register while
feeding the oily dispersion into the first nip, whereby the oily
dispersion upon passing through the first nip forms a film uniformly
covering the surface of the ceramic coating.
According to another preferred embodiment, the at least partially broken
film of oily dispersion is transferred from the first distribution roller
to the second distribution roller and the micro-droplets are transferred
from the second distribution roller to the positive electrode by disposing
a first transfer roller between the first distribution roller and the
second distribution roller in parallel relation thereto, positioning the
first transfer roller in pressure contact engagement with the first
distribution roller to form a second nip and in contact engagement with
the second distribution roller to form a third nip, rotating the first
distribution roller and the first transfer roller in register for
transferring the at least partially broken film from the first
distribution roller to the first transfer roller at the second nip,
disposing a second transfer roller parallel to the second distribution
roller and in pressure contact engagement therewith to form a fourth nip,
positioning the second transfer roller in pressure contact engagement with
the positive electrode to form a fifth nip, and rotating the second
distribution roller, the second transfer roller and the positive electrode
in register for transferring the at least partially broken film from the
first transfer roller to the second distribution roller at the third nip,
then transferring the micro-droplets from the second distribution roller
to the second transfer roller at the fourth nip and thereafter
transferring the micro-droplets from the second transfer roller to the
positive electrode at the fifth nip. Such an arrangement of rollers is
also described in the aforementioned U.S. Pat. No. 5,538,601. Preferably,
the applicator roller, first transfer roller and second transfer roller
are each provided with a peripheral covering of a resilient material which
is resistant to attack by the olefinic substance.
The olefin and metal oxide-coated positive active surface is preferably
polished to increase the adherence of the micro-droplets onto the positive
electrode active surface, prior to step (b) (iii). For example, use can be
made of a rotating brush provided with a plurality of radially extending
bristles made of horsehair and having extremities contacting the surface
of the positive electrode. The friction caused by the bristles contacting
the surface upon rotation of the brush has been found to increase the
adherence of the micro-droplets onto the positive electrode active
surface.
Where the positive cylindrical electrode extends vertically, step (b)(iii)
of the above electrocoagulation printing method is advantageously carried
out by continuously discharging the ink onto the positive electrode active
surface from a fluid discharge means disposed adjacent the electrode gap
at a predetermined height relative to the positive electrode and allowing
the ink to flow downwardly along the positive electrode active surface,
the ink being thus carried by the positive electrode upon rotation thereof
to the electrode gap to fill same. Preferably, excess ink flowing
downwardly off the positive electrode active surface is collected and the
collected ink is recirculated back to the fluid discharge means.
The colloid generally used is a linear colloid of high molecular weight,
that is, one having a molecular weight comprised between about 10,000 and
about 1,000,000, preferably between 100,000 and 600,000. Examples of
suitable colloids include natural polymers such as albumin, gelatin,
casein and agar, and synthetic polymers such as polyacrylic acid,
polyacrylamide and polyvinyl alcohol. A particularly preferred colloid is
an anionic copolymer of acrylamide and acrylic acid having a molecular
weight of about 250,000 and sold by Cyanamid Inc. under the trade mark
ACCOSTRENGTH 86. The colloid is preferably used in an amount of about 6.5
to about 12% by weight, and more preferably in an amount of about 7% by
weight, based on the total weight of the colloidal dispersion. Water is
preferably used as the medium for dispersing the colloid to provide the
desired colloidal dispersion.
The ink also contains a soluble electrolyte and a coloring agent. Preferred
electrolytes include alkali metal halides and alkaline earth metal
halides, such as lithium chloride, sodium chloride, potassium chloride and
calcium chloride. Potassium chloride is particularly preferred. The
electrolyte is preferably used in an amount of about 4.5 to about 6% by
weight, based on the total weight of the dispersion. The coloring agent
can be a dye or a pigment. Examples of suitable dyes which may be used to
color the colloid are the water soluble dyes available from HOECHST such a
Duasyn Acid Black for coloring in black and Duasyn Acid Blue for coloring
in cyan, or those available from RIEDEL-DEHAEN such as Anti-Halo Dye Blue
T. Pina for coloring in cyan, Anti-Halo Dye AC Magenta Extra V01 Pina for
coloring in magenta and Anti-Halo Dye Oxonol Yellow N. Pina for coloring
in yellow. When using a pigment as a coloring agent, use can be made of
the pigments which are available from CABOT CORP. such as Carbon Black
Monarch.RTM. 120 for coloring in black, or those available from HOECHST
such as Hostaperm Blue B2G or B3G for coloring in cyan, Permanent Rubine
F6B or L6B for coloring in magenta and Permanent Yellow DGR or DHG for
coloring in yellow. A dispersing agent is added for uniformly dispersing
the pigment into the ink. Examples of suitable dispersing agents include
the non-ionic dispersing agent sold by ICI Canada Inc. under the trade
mark SOLSPERSE 27000. The pigment is preferably used in an amount of about
6.5 to about 12% by weight, and the dispersing agent in an amount of about
0.4 to about 6% by weight, based on the total weight of the ink.
After coagulation of the colloid, any remaining non-coagulated colloid is
removed from the positive electrode active surface, for example, by
scraping the surface with a soft rubber squeegee, so as to fully uncover
the colored, coagulated colloid. Preferably, the non-coagulated colloid
thus removed is collected and mixed with the collected ink, and the
collected non-coagulated colloid in admixture with the collected ink is
recirculated back to the aforesaid fluid discharge means.
The optical density of the dots of colored, coagulated colloid may be
varied by varying the voltage and/or pulse duration of the pulse-modulated
signals applied to the negative electrodes.
According to a preferred embodiment, the substrate is in the form of a
continuous web which is passed through the respective transfer positions
for being imprinted with the colored images at the printing stages. Step
(c) is preferably carried out by providing at each transfer position a
pressure roller extending parallel to the positive cylindrical electrode
and in pressure contact engagement therewith to form a nip and permit the
pressure roller to be driven by the positive electrode upon rotation
thereof, and guiding the web so as to pass through the nip.
Preferably, the pressure roller is provided with a peripheral covering of a
synthetic rubber material such as a polyurethane having a Shore A hardness
of about 95. A polyurethane covering with such a hardness has been found
to further improve transfer of the colored, coagulated colloid from the
positive electrode active surface onto the substrate. The pressure exerted
between the positive electrode and the pressure roller preferably ranges
from about 50 to about 100 kg/cm.sup.2.
After step (c), the positive electrode active surface is generally cleaned
to remove therefrom any remaining coagulated colloid. According to a
preferred embodiment, the positive electrode is rotatable in a
predetermined direction and any remaining coagulated colloid is removed
from said positive electrode active surface by providing an elongated
rotatable brush extending parallel to the longitudinal axis of the
positive electrode, the brush being provided with a plurality of radially
extending bristles made of horsehair and having extremities contacting
said positive electrode active surface, rotating the brush in a direction
opposite to the direction of rotation of the positive electrode so as to
cause said bristles to frictionally engage the positive electrode active
surface, and directing jets of cleaning liquid under pressure against the
positive electrode active surface, from either side of the brush. In such
an embodiment, the positive electrode active surface and the ink are
preferably maintained at a temperature of about 35.degree.-60.degree. C.
by heating the cleaning liquid to thereby heat the positive electrode
active surface upon contacting same and applying the ink on the heated
electrode surface to cause a transfer of heat therefrom to the ink.
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